Although a channel subsystem is a conventional part of a data processing system and is well known, it will be helpful to review the features and the terminology that particularly apply to this invention. In a simple system, a single computing engine might execute a sequence of instructions that partly use only the resources of the central processor and that partly directly control the resources and the operations of the I/O subsystem. In a system of the type that will use this invention, a central processing engine performs only some of the I/O operations, and several computing engines or processors handle different parts of the I/O operation. One of these processors is called an I/O processor or IOP and there are several processors that are called channel processors or CP's. The CP's handle the actual data transfers into and out of processor memory and they execute commands by forming orders that are sent to the I/O device controllers. The IOP handles general parts of the I/O operation such as communicating with the central processor, scheduling the I/O jobs, and it communicates with the central processor for operations such as getting control blocks that have been prepared by the central processor and reporting status conditions that are handled by the central processor instead of by the IOP.
The IOP and the CP's also communicate with each other, and this invention provides an improvement in these communications. The IOP signals a particular CP when an I/O job is to be performed and the CP signals the IOP when the job has been completed and whenever some other general operation is to be performed by the IOP. In the known prior art, these communications are carried out over signal wires that run between the IOP and the CP's or over the existing data paths that connect the IOP and the CP's to processor storage.